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Filtration steps are involved in many laboratory applications such as sample purification, sample concentration, liquid sterilization, particle sampling, and much more. The properties of your filter determine the success of your filtration step.

Selecting a filter with the correct properties can help you achieve accurate results and reach your goals faster. But with so many types of filters to choose from, how can you be sure to make the right choice?

The tabs below contain information to help you select the correct filter for your application. In addition, our laboratory filtration brochure contains more detailed information.

Hydrophilic
Hydrophilic filters possess an affinity for water and can be wetted with virtually any liquid. They are typically used for aqueous solutions and compatible organic solvents.

HydrophobicHydrophobic filters repel water and are thus best suited for filtering organic solvents, as well as for venting and gas filtration applications.

Loading capacityThe ability of a filter to load particulates into the fibrous matrix while maintaining an acceptablefiltration speed and workable pressure differential across the filter. In general, glass microfiber filters have higher loading capacity compared with cellulose filters, while membranes have inherently low loading capacity.

Particle retention (liquid)In a filtration process, the particle retention efficiency of a depth-type filter is often expressed as the particle size (in µm) which is retained at a set level. The level is usually quoted at 98% efficiency to allow for secondary filtration effects.

Pore size (membranes)Pore sizes are usually stated in micrometers (µm) and are nominal for all membranes apart from Track-Etch and Anopore membranes for which the pore sizes are absolute.

PorosityA measure of how porous a filter material is. For membranes, this is normally expressed as the percentage of the total volume of the filter that is composed of pores. For depth filters, the porosity is often expressed in terms of the flow characteristics in units of time (Gurley porosity).

PrefiltersPlacing a prefilter upstream of a membrane filter considerably reduces the total particulate load, allowing the membrane to operate efficiently and extended its lifetime.

Our qualitative and quantitative cellulose filters are used for general filtration and exhibit particle retention levels down to 2.5 µm. We offer a wide choice of retention/flow rate combinations to meet the needs of numerous laboratory applications.

Borosilicate glass microfibers combine fast flow rates with high loading capacities and retention of very fine particles. Glass microfiber filters must be used flat and should not be folded. Binder-free glass microfiber filters withstand temperatures up to 500ºC and can therefore be used in gravimetric analysis where ignition is involved. Glass microfiber filters exhibit excellent compatibility with organic solvents and strong acids (apart from hydrofluoric acid) and bases. They can be used as prefilters or as final filters, and can retain particles as small as 0.7 µm.

Unlike cellulose and glass microfiber depth filters, membrane filters are conventionally classified as surface filters because the filter matrix retains particles almost entirely on membrane the surface. The narrow effective pore size distribution is one of the major features of Whatman membrane filters, and the retention levels for these filters extend down to 0.02 µm.